In power-generating rotating machines, such as gas turbines or electric generators, the necessary cooling of thermally heavily loaded parts represents an essential physical parameter which has an effect on the overall efficiency and the service life of the system. In most cases, air is used as cooling medium, but steam, which is tapped from a steam generator, can also be used for the same purpose. The present invention, although it is explained by way of example of an air-cooled gas turbine, is not limited to a particular type of cooling and can therefore be used for all types of cooling media.
In FIG. 1, a part of a gas turbine 10 is shown in detail. In this gas turbine 10, intake air, by means of a compressor 11 which comprises a compressor casing 31 and also compressor stator blades 32 and compressor rotor blades 33, is compressed from the ambient pressure to a predetermined operating pressure. After compression, the flow of compressed air is split into compressor-air main flow 12 and a secondary flow 13. After the compressor-air main flow 12 has cooled the hot part of the combustion chamber, it is used in the combustion chamber for combusting a fuel in order to produce hot gas for operating a turbine. The secondary flow 13 of the compressed air is directed from the compressor 11, via cooling passages 14, to the high-temperature region 15 of the gas turbine 10. There, the cooling medium is used for internal cooling of the stator blades 16 and rotor blades 17 of the turbine. In addition, the cooling medium reduces the temperatures on the stator blade fastenings 18 and on the rotating parts, such as on the blade roots 19 and blade necks 20 which, on account of the rotational speed 21 of the rotor 22 around the machine axis 34, are exposed to the extremely high centrifugal forces.
Some of the air is also used for sealing purposes, especially between the rotating and stationary parts of the gas turbine 10, such as between the stator 28 and the rotor 22 (see the sealing systems 23a, 23b and 23c in FIG. 1). In this case, gaps are purged with air which discharges into the hot gas passage (see hot gas main flow 29 in FIG. 1) and so, at this point, prevents the entry of hot gas and consequently local overheating.
It is customary in general to use special devices in this connection, which are referred to as pre-swirl nozzles (24 in FIG. 1), or vortex generators, which are arranged in swirl passages 27, in order to direct the air which is tapped from the compressor 11 to the high-temperature region 15 of the turbine for cooling the rotor 22 and the rotating hot parts, such as the blade roots 19, the blade necks 20 and the blade platforms 25.
Under operating conditions, the thermal load on the hot components of the turbine can decrease or increase, depending upon whether the gas turbine 10 is run under partial load or full load. For example, a reduction of the output power of the gas turbine is customarily brought about as a result of a lowering of the flame temperature in the combustion chamber. Depending upon the demanded power, the gas turbine can be operated at full load and partial load, wherein full load corresponds to the nominal operating conditions. The different operating states are controlled by variable guide vanes (VGV) in the compressor stages, which alter their stagger angle in dependence upon the desired output power. As a result of this, a maximum or lower air mass flow is produced at a constant rotational speed 21.
The magnitude of the flow velocity c of the air downstream of swirl vanes 26 which are arranged in the swirl passages 27 (see FIGS. 1 and 2) is a linear function of the mass flow in the swirl passages 27. For normal operation of the gas turbine, which corresponds to full load, the resulting velocity w is provided by the relationshipw=2π·R·Ω·c, wherein Ω is the rotational speed 21 of the turbine and R is the mean radius at the outlet of the swirl passages 27 (see FIG. 2). The resulting air velocity w influences the total temperature Tt according to the relationshipTtT+w2/(2Cp),wherein T refers to the static temperature and Cp refers to the specific heat.
For a constant rotational speed Ω, the partial load is achieved by means of the variable guide vanes VGV which reduce the mass flow in the compressor 11. Subsequently, the air velocity c downstream of the swirl device (swirl vanes 26) reduces. Ultimately, the resulting velocity w is also influenced by this, which directly affects the metal temperatures of the rotating hot parts, such as the blade roots 19, the blade necks 20 and the platforms 25. If the metal temperature at constant rotational speed is kept constant, the corresponding mechanical components are not exposed to low cycle fatigue (LCF). This could technically be achieved by means of controlled valves. In actual fact, however, the swirl device is not usually provided with control elements, which can influence the mass flow in the cooling passages 14 since this region of the rotor 22 and the stator 28 is accessible only to a limited degree.
Controlling the cooling air distribution in the rotor 22, in the stator 28 and in the turbine blades 27 is a complicated undertaking, which is additionally made more difficult as a result of the requirement for avoiding backflows. Resulting from this is the fact that a simple throttling does not represent a satisfactory solution and that it is advantageous to use a control device with an aerodynamically optimized design. Such a device is the pre-swirl nozzle 24 which is customarily formed by means of a stationary row of blade airfoils in the style of turbine guide vanes (swirl vanes 26 in FIG. 3a). These swirl vanes 26 are fastened on the stator 28 between the compressor 11 and the high-temperature region 15. A constriction 35 (FIG. 3b), with a corresponding area F (FIG. 3c), is formed between the swirl vanes 26.
If a simple, functionally reliable, automatic control of the mass flow could be realized in a simple manner in the region of the pre-swirl nozzle 24, a particularly effective cooling of the corresponding regions at different load states of the turbine could be realized without great cost.
GB 2 354 290 describes controlling the cooling air flow through the inside of a turbine blade in a gas turbine by means of a circular valve consisting of a shape-memory alloy.
A similar solution, in which sleeves consisting of a shape-memory alloy are inserted in the individual disks of the turbine and alter the cross section of cooling medium passages in dependence upon temperature, is described in printed publication US 2009/0226327 A1.
In both cases, it concerns the main flow of cooling medium.
Furthermore, the printed publication GB 2 470 253 describes a device for controlling the cooling medium flow in a gas turbine. An annular flow limiter, which is provided with apertures arranged distributed over the circumference is used. The flow cross section of the apertures can be changed in each case by a valve element, the position of which in relation to the aperture is changed by means of an SMM element.
The printed publication US 2002/076318 describes the tangential injection of cooling air from outside into the rotor of a gas turbine for cooling the rotor blades. The injection takes place by mixing two separate flows, one of which is emitted from inside injector blades provided for the injection. Control by changing the cross section, in particular using a shape-memory alloy, is not disclosed.